At the risk of sounding like a parrot: Take the meds fluffy! I'm afraid I don't know what PICU or section 3 is, but taking them yourself, even if you don't think they are worth anything, has got to be better than an injection!
spidey stop with the punishments! As you say, the time to punish him for biting is right after it happens. A simple no, we don't bite. Stick in time out. Then at the end explain what he did was wrong, it hurts, makes the person sad and finish. There is just no point you punishing him once he gets home. Is there anything he sees/does at home that could be influencing his behaviour. We let DS watch tom and jerry. Huge mistake. I noticed a distinct improvement in his behaviour when I said he wasn't allowed to watch it anymore. Same with the iPad, I banned it for a week because he threw it on the floor when I asked him to stop and come to tea. His change in his behaviour was dramatic, he has barely had it the past month and he no longer asks for it. He is less violent. What also helped us was a glow In the dark star chart. At first it was for small things, doing as asked, getting shoes on when asked etc. When he goes to bed we lie and look at the stars and see how many he has and say how nice it was he did x and thank him, or how proud of what he has done. When he gets a certain number (started simple, with 5) he took them off and in exchange he got a present. Then gradually increased the number if stars and expectations. Now he gets a star if I have a good report at the end of the day from school, he's kind to someone or helps with something. He loves it. And it is such a change from last year (when he was 3!) no more hitting, kicking etc. If he's had a bad report (like yesterday when he destroyed another child's creation) then I say how disappointed I am to hear what he did, tell him it wasn't kind to upset the other child and that today he won't be getting a star. And then say nothing more. It's far more effective for us than confiscating toys etc.
We made a conscious decision not to punish/reward with food because we didn't want them to associate sweet things as being a reward, rather that it's something that we sometimes eat in small amounts after a meal. So unless one makes a mess at the table then they still get a pudding.
Three is really, really little. Give him time. He is still learning.